The present invention relates to optical semiconductor devices, and particularly, to a structure for improving the efficiency of light emitters and photodetectors fabricated from GaN-based semiconductors.
In the following discussion a III-N semiconductor is a semiconductor having a Group III element and nitrogen. III-N semiconductors such as GaN are useful in fabricating light emitting elements that emit in the blue and violet regions of the optical spectrum. These elements include light emitting diodes and laser diodes. Laser diodes that use semiconductor material based on GaN that emit in the blue and violet regions of the spectrum hold the promise of substantially improving the amount of information that can be stored on an optical disk. However, higher efficiencies are needed for both semiconductor light emitters and photodetectors. This is a particularly urgent problem in GaN-based optical semiconductor devices using BN, AlN, GaN, or InN, which are compounds of nitrogen and Group III elements such as B, Al, Ga, and In and their mixed crystal semiconductors (hereinafter, called GaN-based semiconductors).
Light emitting elements based on III-N semiconductors are typically fabricated by creating a p-n diode structure having a light generating region between the p-type and n-type layers. The diode is constructed from layers of III-N semiconducting materials. After the appropriate layers are grown, electrodes are formed on the p-type and n-type layers to provide the electrical connections for driving the light-emitting element.
One class of blue and green light-emitting diodes (LEDs) or short-wavelength laser diodes (LDs) use GaInN/GaN strained quantum wells or GaInN/GaInN strained quantum wells located between the n-type and p-type layers to generate light by the recombination of holes and electrons injected from these layers. In prior art devices, a strained GaN-based semiconductor layer is constructed by growing a {0001} plane of a normal GaN-based crystal. The resulting layer has a large piezoelectric field. For example, in a Ga0.9In0.1N strained layer, an extremely large piezoelectric field of around 1 MV/cm is generated.
Usually, when an electric field exists in a quantum well, the energy band of the quantum well layer tends to tilt substantially as the electric field increases. As a result, the wave functions of the electrons and holes separate from one another, and the overlap integrals of both wave functions decrease. Since the optical properties such as the light emission and absorption efficiencies depend on these overlap integrals, the efficiency of these devices decreases with increasing electric fields.
Broadly, it is the object of the present invention to provide an improved III-N semiconductor device in which the efficiency of light generation or detection is increased relative to prior art devices.
It is a further object of the present invention to provide a strained quantum well layer having a reduced piezoelectric field.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is an optical semiconductor device having a plurality of GaN-based semiconductor layers containing a strained quantum well layer in which the strained quantum well layer has a piezoelectric field that depends on the orientation of the strained quantum well layer when the quantum layer is grown. In the present invention, the strained quantum well layer is grown with an orientation at which the piezoelectric field is less than the maximum value of the piezoelectric field strength as a function of the orientation. In devices having GaN-based semiconductor layers with a wurtzite crystal structure, the growth orientation of the strained quantum well layer is tilted at least 1xc2x0 from the {0001} direction of the wurtzite crystal structure. In devices having GaN-based semiconductor layers with a zincblende crystal structure, the growth orientation of the strained quantum well layer is tilted at least 1xc2x0 from the {111} direction of the zincblende crystal structure. In the preferred embodiment of the present invention, the growth orientation is chosen to minimize the piezoelectric field in the strained quantum well layer.